Hydrocarbon Processing - June 2022 - GP-22

LNG
of CO2
ture needs a far smaller CO2
(20%-30%). Pre-combustion caprecovery
unit
but requires other additional equipment to
process the hydrocarbon fuel gas into clean
H2
for use in the gas turbines. Although
not directly used for LNG service, the
required technology for pre-combustion
capture has been used for decades. For example,
the authors' company's Port Arthur
CO2
mately 1 MM metric tpy of CO2
large-scale H2
capture project has captured approxifrom
two
plants since it commenced
operations in early 2013.
The advantages of pre-combustion
capture allow for easier retrofit into existing
plants and easier siting of the additional
equipment needed for CO2
capture.
The proprietary pre-combustion capture
processa
to capture the CO2
in the gas turbine flue gas.
Pre-combustion capture technology
overview. The proprietary process
consists of three major systems (shown
in FIG. 3):
1. The LNG liquefaction plant,
including pretreatment
Air
CH4
Flue gas with CO2
CO2 capture
Low pressure
uses a pre-combustion approach
that would be released
2. The H2
production facility
3. The power generation system.
The LNG liquefaction plant takes in
raw natural gas feed. Gas pretreatment, including
acid gas removal, mercury removal
and any heavy component removal, occurs
upstream of the liquefaction plant. The
liquefaction plant produces LNG from the
pretreated gas while also producing a fuel
stream that consists of end-flash and boiloff
gas. Normally, this fuel stream would be
sent directly to the gas turbines to generate
power. However, in the proprietary precombustion
capture process, the fuel gas
stream, which is primarily methane (CH4
and nitrogen (N2
)
), is instead sent to an H2
production plant. The specific technology
used for H2
options are discussed below.
In the H2
production can vary; potential
production facility, the fuel
gas stream is combined with oxygen
(or air) and steam. The combined stream
is sent to a series of reactors to generate
a syngas stream of primarily H2
The syngas stream contains CO2
and CO2
at high
.
pressure (> 30 bar) and at a high fraction
of the total stream molar flow (20%-
30%). These conditions are well suited
CO2 to sequestration
Flue gas
for CO2
separation from H2
moderate cost.
CO2
process. First, the CO2
product at
is removed at two points in the
in the natural
gas feed is removed in the AGRU. Next,
CO2
generated during the reforming and
water-gas shift reactions within the H2
production facility is removed during a
downstream separation step. This separation
can be achieved through an amine
unit similar to the AGRU, or through vacuum
swing adsorption (VSA), cryogenic
distillation or other techniques. Several of
these technologies can achieve the target
of > 97% CO2
After CO2
recovery.
removal, H2
duction facility is combined with CH4
used as pure H2
from the proor
fuel.
The fuel is mixed
) formation and is sent
with one or more diluents to suppress nitrogen
oxide (NOx
to the power generation system, which
produces power for liquefaction and the
H2
production facility.
The power generation system may be
simple or combined-cycle, and it may either
be directly coupled to the liquefaction
compressors or instead coupled to generators
to produce electricity used by electric
motors in the liquefaction facility.
Hydrogen production facility options.
The pre-combustion capture processa
use a wide variety of technologies for its
H2
can
production plant, including the steam
Gas turbine
Compressor
methane reforming (SMR) process, the
autothermal reforming (ATR) process, or
the partial oxidation (POX) process. All
three options include a main reactor, one
or more water-gas shift reactors, and several
heat exchangers for steam generation.
The SMR process uses a tubular cataFIG.
1. Generic configuration for post-combustion CO2
O2
CH4
H2O
H2 generation
pressure
Syngas
High
capture.
CO2 to sequestration
CO2 capture
Air
H2
N2,O2,H2O
lytic main reactor where fuel is burned
at low pressure externally to the tubes to
provide the heat needed for the reforming
reaction. The SMR process is an industry
standard for gray H2
production thanks to
its energy efficiency, high reliability and
attractive capital cost. However, because
fuel is burned at low pressure, approximately
40% of the total CO2
in the H2
generated
production facility is released
in low-pressure flue gas and cannot be recovered
through pre-combustion capture.
The POX and ATR processes instead
Gas turbine
FIG. 2. Generic configuration for pre-combustion CO2
22 MAY/JUNE 2022 | GasProcessingNews.com
capture.
Compressor
provide the required heat for their reactions
by reacting a portion of the feed
stream within the reactor vessel. This
allows all CO2
formed during the reactions
to remain within the high-pressure
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Hydrocarbon Processing - June 2022

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